1. Field of the Invention
The present invention relates to polyolefin foam beads, and more particularly to polyolefin foam beads having a high-density crust and a method for manufacturing the same.
2. Description of the Related Art
Polymer foams made from polyurethane, polystyrene, or polyolefin are widely used as a shock absorbing material, heat insulating material, and packing material due to their flexibility, durability, light weight, and low thermal conductivity.
Generally, polyolefin foam products are manufactured by 2 steps. First, polyolefin foam beads are prepared by vaporizing foaming agent impregnated in resin particle, then final foam products having desired shape are manufactured by molding the foam beads under heat.
When the expansion ratio of the foam bead increases, the final foam product becomes lighter and consumes less resin, while the strength and the dimensional stability decreases. In order to improve the strength and the dimensional stability, the expansion ratio of the foam bead should be lowered and the foam product becomes heavier and consumes more resin.
The object of the present invention is to provide polyolefin foam bead from which final foam product of higher strength and higher dimensional stability can be obtained at higher expansion ration.
In order to achieve the above object, the present invention provides polyolefin foam bead having a surface layer (crust) which is higher in density than its inner structure and a method for manufacturing the same. The method comprises the step of impregnating a polyolefin resin particle with a forming agent by stirring the mixture of resin particles, foaming agent and water in a high pressure vessel under heat, followed by diffusing out the foaming agent in a surface layer (crust) to decrease the concentration of the foaming agent in the surface layer. Thereafter, the resin particles are discharged to a low pressure environment to obtain the foam beads.